LONDON—A groundbreaking deal to curb Iran's nuclear program faces towering obstacles at home and abroad to becoming a permanent agreement, starting with the U.S. Congress and two of America's closest allies.

The leaders of both the Democratic and Republican parties are threatening to break with President Barack Obama's policy and enact new punitive sanctions on Iran, arguing that the interim deal reached in Geneva on Sunday yields...

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Israel Sharply Opposed to Iran Nuclear Deal

Israeli leaders, watching with trepidation over Iran's interim nuclear agreement, are wrangling over how to ensure the next rounds of diplomacy yield the best possible result for their country. Washington Institute senior fellow Andrew Tabler discusses. Photo: Getty Images.